Measurement of the timing and characteristics of photonic emissions is critical to many scientific applications. Photons are characterized by their wavelength, their polarization, and by their location in time and space.
A single photon detector (SPD) is a device that produces an electrical signal when a single photon is absorbed by the detector. SPD's include a single photon detector component that absorbs the photon and undergoes a change in state that produces the electrical signal. Recently single photon detector components have taken the form of solid state microelectronic circuits made using fabrication methods known in the art of solid state microelectronics. SPD devices also include optical components necessary to direct the light to the SPD microelectronic circuit and electrical components to amplify and process the electrical signals produced by the microelectronic circuit.
SPD's are widely used in scientific research in the fields of medicine, biology, astronomy, physics, chemistry, electrical and chemical engineering, material science, and aeronautics. Additionally, single-photon detectors are an essential tool for a wide range of applications in quantum information, quantum communications, and quantum optics.
The effectiveness of a photon detection device is measured in terms efficiency. Ideally, the detector should produce a signal every time a single photon enters the device. The probability that an electrical signal will be produced when a photon enters the SPD (expressed as a percent) is referred to as the system efficiency. The probability that a photon contacting the SPD microelectronic circuit will produce an electrical signal is known as the quantum efficiency of the microelectronic circuit. The wavelength, polarization, and the position of the light all affect the system efficiency of the SPD device, and quantum efficiency of the microelectronic circuit. The system efficiency can be no greater, and is frequently less, than the quantum efficiency of the microelectronic circuit.
Other characteristics of SPD's used to determine their suitability for certain applications are: signal-to-noise ratio, timing jitter, reset times, and dark count rate. Signal to noise ratio is the ratio of magnitude of the electrical signal produced from the device to the magnitude of electrical noise of the device. Timing jitter is the deviation of the measured photon arrival time from the actual photon arrival time. Reset time is the time it takes for the device to be reset to receive another photon after an initial photon is detected. Dark count rate is the number of (false) detection signals produced per unit time when no photons are entering the device. For SPD's to be useful in the widest possible applications they should have high signal-to-noise, low timing jitter, fast reset times, and low dark count rates.
There are numerous types of photon detector microelectronic circuits known in art. One type of SPD known in the art is an avalanche photodiode (APD). APD's are highly sensitive semiconductor electronic devices that exploit the photoelectric effect to convert light into electricity. An APD can be thought of as a photo-detector with a built-in first stage of gain through avalanche multiplication. From a functional standpoint, they can be regarded as the semiconductor analog to photomultipliers. Over the past decade, superconducting nanowire single photon detectors (SN-SPDs) have become promising alternatives to conventional semiconductor avalanche photodiodes in the near-infrared region of the spectrum. In particular, SN-SPDs based on niobium nitride (NbN) superconducting nanowires have demonstrated desirable properties 30 picoseconds timing jitter, fast reset times on the order of 10 nanoseconds, and low dark count rates below 1 kcps (thousand counts per second), but generally suffer from low system detection efficiencies of less than 20%. In addition, the efficiency of these detectors depends strongly on the polarization of the incident light.
Another problem known in the art is that the quantum efficiency of SN-SPD's degrades as the width of the nanowires increase, and therefore microelectronic circuits in the prior art have been limited to the use of extremely narrow nanowires. The quantum efficiency and sensing area of the microelectronic circuit are affected by the width of the nanowires and the width of channel spaces between the nanowires. NbN nanowires known in the art are arranged in a continuous meander pattern across the surface of the detector microelectronic circuit. In theory, larger geometric area covered by the nanowires should translate into a proportionately larger quantum efficiency of the microelectronic circuit. However, nanowires frequently have width dimensions of the order of the channels separating the nanowires from each other. Nanowires may have an approximate thickness of 5 nm and an approximate width of 100 nm. Current electron beam fabrication methods make it extremely difficult to create nanowires and channel features smaller than 100 nanometers. A limiting factor governing the quantum efficiency of SN-SPD microelectronic circuits based on NbN is that they have a relatively small area where absorption of photons may take place.
Another problem known in the art is that SPD devices experience decreased quantum efficiencies for photons having longer wave lengths. SPD devices currently known in the art, which utilize NbN, have achieved extremely high quantum efficiencies for photons whose wavelength is in the UV, visible, and for some wavelengths in the near infrared region of the spectrum.
Another problem known in the art is that the detection efficiency of SNSPD's depends upon the polarization of the light. There is an unmet need for superconducting SPDs that can detect photons with close to 100% efficiency for any photon wavelength, and any photon polarization. It is desirable to have SN-SPD technologies that provide larger areas for the absorption of photons within a designated physical area of the microelectronic circuit. It is desirable to have SN-SPD's with high signal-to-noise.